This invention relates to a flexible magnetic disk, in which an improved surface vibration of a recording medium is obtained when the disk is rotated at a high speed.
Recently, there have been proposed electronic still video systems in which a camera device such as a solid state camera device or a camera tube is combined with a magnetic recording apparatus using an inexpensive flexible magnetic disk with a relatively large recording capacity. The proposed electronic still video systems record still image information of a subject on the rotating magnetic disk and reproduce the still image by means of a television system or a video printer.
The flexible magnetic disk used in the systems includes a flexible polymer substrate such as polyethylene terephthalate, polyethylene naphthalate, polyimide, polyphenylene sulfide or the like. A magnetic layer is formed on the substrate by applying a magnetic coating material comprising a metallic magnetic powder dispersed in an organic binder, where the magnetic layer is then dried upon application. The coated substrate is processed into a circular medium and encased in a resin shell such as ABS. The resulting product is generally referred to as a video floppy disk.
In the electronic still video system, a uniform spacing is maintained between the magnetic disk, which rotates at a high speed of 3,600 r.p.m., and a magnetic head. The spacing must be sufficiently small in comparison to a shortest central recording wavelength benchmark of 0.76 .mu.m. Conventional rotating magnetic disks sometimes vibrate in widths of tens to several hundred .mu.m due to a curling or an irregularity of the medium, thereby making it difficult to maintain a uniform space between the disk surface and the head.
Generally, only one side of the magnetic disk employed in the electronic still video system is used for recording and reproduction. However, to maintain a proper curl balance both sides of the magnetic disk are provided with a magnetic layer of the same thickness, so that the total thickness is approximately 40.+-.2 .mu.m. Furthermore, formation of the magnetic layer is achieved by successive coatings, where calendering is performed on the first coating surface or both coating surfaces to obtain a smooth surface.
With the trend being toward compact design of the still video systems, the drive mechanisms are required to be thinner in structure. Specifically, conventional head touch mechanisms are being changed from the known positive pressure types (where a fixed pad is disposed at the rear side of the disk to press and hold the head against the disk or a positive pressure is generated by the air flow) to negative pressure types (where the disk is attracted towards the head using a negative pressure generated by the air flow). Also, there is an increasing demand for an improved magnetic disk which maintains a low surface vibration during rotation of the disk at high speeds.
With a view to eliminate the above known problems, it is a primary object of the present invention to provide a flexible magnetic disk in which an improved surface vibration of the recording medium is obtained at high-speed rotation of the magnetic disk.